The present invention relates to a process for preparing substantially gel-free copolymers of 1,3-butadiene and isobutylene. More particularly, the invention relates to a homogeneous process for preparing such copolymers, utilizing a catalyst system consisting of BF.sub.3 and ROH (defined hereinafter).
Copolymers of the type embodied in this invention are commonly referred to in the prior art as butyl rubbers. This term generally refers to copolymers prepared by the polymerization of isobutylene with a conjugated diene. Early processes for preparing such copolymers generally resulted in the production of copolymers having low levels of unsaturation, i.e., 1.5 mole percent or less. In addition, at higher unsaturation levels these early copolymers often exhibited a low number average molecular weight and in many instances a very high gel content.
Thus, U.S. Pat. No. 2,607,764 to Nelson is directed to a copolymer of isobutylene and butadiene which is prepared by polymerizing a mixture of butadiene and isobutylene, in a ratio of butadiene to isobutylene of between 2 to 10 or 12 times as much butadiene as isobutylene, in a halo-substituted aliphatic, carbon disulfide or low-freezing hydrocarbon solvent using a Friedel-Crafts catalyst such as a metal halide, e.g., aluminum chloride at a temperature below -6.degree. C. The patent discloses that the preferred polymerization temperature lies below -6.degree. C. and preferably within the range between about -40.degree. C. and -103.degree. to -110.degree. C. but may be as low as -164.degree. C. However, the patent indicates a number of the copolymers described therein contain significant amounts of gel as shown in Example 2, Table III at column 11 of the patent wherein it is disclosed that isobutylene-butadiene copolymers of high unsaturation are not completely soluble in hydrocarbons.
U.S. Pat. No. 2,844,569 to Green et al describes a process for preparing copolymers of diolefin, including butadiene, and isobutylene which involves polymerizing the monomer in a saturated hydrocarbon solvent at a polymerization temperature of from 0.degree. C. to -164.degree. C., preferably from -35.degree. C. to -103.degree. C., in the presence of a Friedel-Crafts catalyst dissolved in a low-freezing, non-complex forming solvent. Friedel-Crafts catalysts which are disclosed include among others aluminum chloride and boron trifluoride. However, applicants have found that the use of BF.sub.3 as the sole catalyst in a very dry polymerization system is not very effective, often resulting in low yields or lengthy polymerization times. In addition, the patent at column 4, lines 47-49 states that use of lower alcohols in the polymerization system should be avoided on the grounds that such alcohols are not solvents for the polymer and also react with the catalyst.
U.S. Pat. No. 2,775,537 to Wilson is directed to cements for bonding together dissimilar polymers. The cements are prepared by polymerization of isobutylene and a diolefin including butadiene utilizing a Friedel-Crafts catalyst. However, the patent indicates that the resulting copolymers have low molecular weights or have high levels of gel.
U.S. Pat. No. 3,466,268 to Barton et al and its parent counterpart U.S. Pat. No. 3,357,960 discloses butadiene-isobutylene copolymers and a method for their preparation. The invention disclosed in these references is directed to a method of improving butadiene polymers by incorporating varying amounts of isobutylene in the polymer. Preferably, the amount of isobutylene incorporated is said to be about 2 to 40 weight percent. However, the copolymers disclosed therein are low in number average molecular weight.
More recently, attempts have been made in the prior art to produce substantially gel-free copolymers of isoolefins and conjugated dienes having a mole percent unsaturation of at least 5% and a number average molecular weight of at least 120,000.
Thus, for example, U.S. Pat. No. 3,928,297 to Thaler et al describes the preparation of such copolymers by a process which involves the polymerization of the monomers in not more than 40 weight percent of a cosolvent which is a solvent for the polymer at the polymerization temperature in the presence of a metal halide (e.g., aluminum chloride) at a temperature of less than -100.degree. C. The catalyst is introduced into the reaction zone in a polar solvent. The patent specifically excludes the use of butadiene.
U.S. Pat. No. 3,856,763 to Thaler et al describes the preparation of such copolymers using a homogeneous polymerization system featuring a hydrocarbyl aluminum dihalide catalyst. The reaction is carried out in a solvent mixture consisting of paraffinic solvents at a temperature of -100.degree. C. Again, the patent excludes butadiene.
U.S. Pat. No. 3,808,177 at Thaler et al describes a similar process for producing such copolymers in which the catalyst is an aluminum halide and the reaction is carried out in a solvent medium consisting of a polar solvent, e.g., a halogenated aliphatic hydrocarbon and a cosolvent consisting of a paraffinic hydrocarbon such as pentane, isopentane, cyclohexane, heptane, etc. Again, butadiene is specifically excluded.